Date of Award

12-2012

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Alfredo Galindo-Uribarri

Committee Members

Edmund Perfect, Yingkui Li

Abstract

The measurement of cosmogenic 26Al [aluminum-26] in geological samples by accelerator mass spectrometry (AMS) is typically conducted on Al2O3 [aluminum oxide] targets. However, Al2O3 is not an ideal source material because it does not form a prolific beam of Al- [negative atomic aluminum ions] required for measuring low-levels of 26Al. This thesis presents the performance of AlN [aluminum nitride], AlF3 [aluminum fluoride] and mixed AlN + Al2O3 as novel alternative source materials for the analysis of 26Al. A negative ion cesium sputtering source at the Holifield Radioactive Ion Facility was used to measure the currents of stable atomic 27Al- ions as well as molecular AlX- ions of commercially prepared target samples. Here it is shown that an AlN target produces an Al- current seven times greater than that of an Al2O3 target and a molecular AlN- current that is four times greater. The performance of AlN in producing negative ion beams is shown to be dependent on the length of exposure to moist air, which is known to cause AlN to hydrolyze to Al(OH)3. A peak in performance is observed after one hour of exposure. This suggests that the formation of an intermediary product of hydrolysis, such as AlOOH, may increase the ionization efficiency of the AlN material. The AlF3 and mixed AlN + Al2O3 targets did not yield prolific ion beams of Al species and therefore were not promising source materials. The applicability of using AlN as a source material for geological samples was explored by preparing quartz samples as Al2O3 and converting them to AlN using a carbothermal reduction technique, which involves reducing the Al2O3 with graphite powder at 1600°C within a nitrogen atmosphere. The material was successfully converted to AlN and yielded an atomic Al- current higher than the Al2O3 sample. However, a large excess of carbon bonded with the aluminum in the sample forming AlC2 [aluminum carbide] and inhibited the production of AlN. While AlN represents a promising source material for the analysis of 26Al, further work is needed to optimize the conversion process for geological samples.

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